Radioimmunoassay techniques have been developed so that cortisol, progesterone, DHAS and total estriol can all be assayed in 0.5 ml serum. These steroids are being assayed in serial samples from sick premature infants in order to determine their values, if any, in predicting the outcome of diseases of neonatal adaptation. In addition, differences in hormonal status between normally grown newborns and growth retarded newborns are being examined.